In this context, one device already proposed allows one to diagnose various respiratory disorders, such as apnea, revealing a pathology known by the name of Syndrome of Sleep Apnea (“SSA”). Such an apnea is defined as a respiratory pause of a duration longer than ten seconds, and occurring during a phase of sleep of the patient. SSA can also be defined by a significant recurrence of hypopneas. One can also detect disorders revealed by a periodic and abnormal variation of the respiratory profile, where phases of hyperventilation alternate with either phases of normal breathing or phases of hypoventilation, a disorder known by the name of “Periodic Breathing (“PB”), or respiratory pause phases (a disorder known by the name of “Cheyne-Stokes dyspnea” or “Cheyne-Stokes breathing”, or Cheyne-Stokes Respiration (“CSR”)). The various alternate phases each have a duration of a few respiratory cycles to a few tens of respiratory cycles, i.e., of a few seconds or tens of seconds, and in some circumstances in excess of a minute.
Cardiac pacemakers including means for measuring the respiratory activity of the patient, more precisely the minute-ventilation (i.e., the product of the respiratory frequency by the ventiliatory amplitude) are known from, for example, European Patent Publication EP-A-0 493 222 and its corresponding U.S. Pat. No. 5,303,702, assigned to the assignee hereof Ela Médical. However, the minute ventilation parameter is disclosed in that document to control the frequency of application of the stimulation pulses, but not to carry out an analysis of the ventilation able to detect some particular disturbance occurring during sleep.
The starting point of the invention lies in the observation of the existence of respiratory disorders occurring during sleep, the symptoms of which are neither respiratory pauses nor periodic variations of the ventilatory profile. These disorders can nevertheless-induce, in particular, an oxygen desaturation of the blood that is likely to cause the patient to wake up, and further, especially, a transitory sinusal tachycardia intended for compensating the desaturation. Those disorders are called “hypopneas” (as defined below).